The invention relates generally to polymeric positive temperature coefficient (PTC) compositions and electrical PTC devices. In particular, the invention relates to polymeric PTC compositions containing surface treated kaolin clay which exhibits improved over voltage capabilities and an enhanced PTC effect.
Electrical devices comprising conductive polymeric compositions that exhibit a PTC effect are well known in electronic industries and have many applications, including their use as constant temperature heaters, thermal sensors, low power circuit protectors and over current regulators for appliances and live voltage applications, by way of non-limiting example. A typical conductive polymeric PTC composition comprises a matrix of a crystalline or semi-crystalline thermoplastic resin (e.g., polyethylene) or an amorphous thermoset resin (e.g., epoxy resin) containing a dispersion of a conductive filler, such as carbon black, graphite chopped fibers, nickel particles or silver flakes. Some compositions additionally contain flame retardants, stabilizers, antioxidants, antiozonants, accelerators, pigments, foaming agents, crosslinking agents, dispersing agents and inert fillers.
At a low temperature (e.g. room temperature), the polymeric PTC composition has a contiguous structure that provides a conducting path for an electrical current, presenting low resistivity. However, when a PTC device comprising the composition is heated or an over current causes the device to self-heat to a transition temperature, a less ordered polymer structure resulting from a large thermal expansion presents a high resistivity. In electrical PTC devices, for example, this high resistivity limits the load current, leading to circuit shut off. In the context of this invention Ts is used to denote the xe2x80x9cswitchingxe2x80x9d temperature at which the xe2x80x9cPTC effectxe2x80x9d (a rapid increase in resistivity) takes place. The sharpness of the resistivity change as plotted on a resistance versus temperature curve is denoted as xe2x80x9csquarenessxe2x80x9d, i.e., the more vertical the curve at the Ts, the smaller is the temperature range over which the resistivity changes from the low to the maximum values. When the device is cooled to the low temperature value, the resistivity will theoretically return to its previous value. However, in practice, the low-temperature resistivity of the polymeric PTC composition may progressively increase as the number of low-high-low temperature cycles increases, an electrical instability effect known as xe2x80x9cratchetingxe2x80x9d. Crosslinking of a conductive polymer by chemicals or irradiation, or the addition of inert fillers or organic additives may be employed to improve electrical stability.
In the preparation of the conductive PTC polymeric compositions, the processing temperature often exceeds the melting point of the polymer by 20xc2x0 C. or more, with the result that the polymers may undergo some decomposition or oxidation during the forming process. In addition, some devices exhibit thermal instability at high temperatures and/or high voltages that may result in aging of the polymer. Thus, inert fillers and/or antioxidants, etc. may be employed to provide thermal stability.
Among the known inert fillers employed in PTC polymeric compositions are polymeric powders such as polytetrafluoroethylene (e.g., Teflon(trademark) powder), polyethylene and other plastic powders, fumed silica, calcium carbonate, magnesium carbonate, aluminum hydroxide, kaolin clay, talc, chopped glass or continuous glass, fiberglass and fibers such as Kelvar(trademark) polyaramide fiber (available from DuPont) among others. According to U.S. Pat. No. 4,833,305 by Machino et al., the fibers employed preferably have an aspect ratio of approximately 100 to 3500, a diameter of at least approximately 0.05 microns and a length of at least approximately 20 microns.
Polymeric PTC materials have found a variety of applications, such as self-regulating heaters and self-resettable sensors to protect equipment from damage caused by over-temperature or over-current surge. For circuit protection, the polymeric PTC devices are normally required to have the ability to self-reset, to have a low resistivity at 25xc2x0 C. (10 xcexa9cm or less), and to have a moderately high PTC effect (103 or higher) in order to withstand a direct current (DC) voltage of 16 to 20 volts. Polyolefins, particularly polyethylene (PE)-based conductive materials, have been widely explored and employed in these low DC voltage applications.
Polymeric PTC sensor devices that are capable of operating at much higher voltages, such as 240 alternating current voltages (VAC) (xe2x80x9cLinexe2x80x9d voltages) present in AC electrical lines. PTC devices with high voltage capabilities have been found to be particularly useful as self-resettable sensors to protect AC motors from damage caused by over-temperature or over-current surge. For example, and without limitation, such high voltage capacity polymeric PTC devices would be useful to protect the motors of household appliances, such as dishwashers, washers, refrigerators and the like.
In view of the foregoing, there is a need for the development of polymeric PTC compositions and devices comprising them that exhibit a high PTC effect, have a low initial resistivity, that exhibit substantial electrical and thermal stability, and that are capable of use over a broad voltage range, i.e., from about 6 volts to about 300 volts.
The invention provides polymeric PTC compositions and electrical PTC devices having increased voltage capabilities while maintaining a low RT resistance. In particular, the polymeric compositions also demonstrate a high PTC effect (the resistivity at the Ts is at least 103 times the resistivity at 25xc2x0 C.) and a low initial resistivity at 25xc2x0 C. (preferably 10 xcexa9cm or less, more preferably 5 mxcexa9 or less). The electrical PTC devices comprising these polymeric PTC compositions preferably have a resistance at 25xc2x0 C. of 500 mxcexa9 or less (preferably about 5 mxcexa9 to about 500 mxcexa9, more preferably about 7.5 mxcexa9 to about 200 mxcexa9, typically about 10 mxcexa9 to about 100 mxcexa9) with a desirable design geometry.
The polymeric PTC compositions of the invention, demonstrating the above characteristics include a predetermined amount of surface treated kaolin clay. More particularly, the PTC compositions comprise an organic polymer, a particulate conductive filler, an inorganic additive including surface treated kaolin clay, and, optionally, an additive selected from the group consisting of inert fillers, flame retardants, stabilizers antioxidants, antiozonants, accelerators, pigments, foaming agents, crosslinking agents and dispersing agents.
The electrical PTC devices of the invention have, for example, the high voltage capability to protect equipment operating on Line current voltages from over-heating and/or over-current surges. The devices are particularly useful as self-resetting sensors for AC motors, such as those of household appliances, such as dishwashers, washers, refrigerators and the like. Additionally, PTC compositions for use in low voltage devices such as batteries, actuators, disk drives, test equipment and automotive applications are also described below.